In operation of a high-intensity metal halide discharge lamp, visible radiation is emitted by the metal portion of the metal halide fill at relatively high pressure upon excitation typically caused by passage of current therethrough. One class of high-intensity, metal halide lamps comprises electrodeless lamps which generate an arc discharge by establishing a solenoidal electric field in the high-pressure gaseous lamp fill comprising the combination of one or more metal halides and an inert buffer gas. In particular, the lamp fill, or discharge plasma, is excited by radio frequency (RF) current in an excitation coil surrounding an arc tube which contains the fill. The arc tube and excitation coil assembly acts essentially as a transformer which couples RF energy to the plasma. That is, the excitation coil acts as a primary coil, and the plasma functions as a single-turn secondary RF current in the excitation coil produces a time-varying magnetic field, in turn creating an electric field in the plasma which closes completely upon itself, i.e., a solenoidal electric field. Current flows as a result of this electric field, thus producing a toroidal arc discharge in the arc tube.
High-intensity, metal halide discharge lamps, such as the aforementioned electrodeless lamps, generally provide good color rendition and high efficacy in accordance with the principles of general purpose illumination. However, the lifetime of such lamps can be limited by the loss of the metal portion of the metal halide fill during lamp operation and the corresponding buildup of free halogen. In particular, during lamp operation, the metal halide fill is dissociated by the arc discharge into positive metal ions and negative halide ions. The positive metal ions are driven toward the arc tube wall by the electric field of the arc discharge. Metal which does not react with halide ions before reaching the arc tube wall may react chemically at the wall. For example, in an arc tube containing a fill including sodium iodide and cerium iodide, sodium reacts with the quartz arc tube to form sodium silicate crystals, causing devitrification of the arc tube. Moreover, the dose of sodium and cerium iodides catalyzes the crystal nucleation of fused silica, enhancing the devitrification process. The thermal mismatch between the newly formed crystalline silica and the amorphous silica of the arc tube causes severe cracking of the arc tube wall. As another problem, cerium causes chemical etching of the arc tube wall, leading to rough and uneven inner wall surfaces. Furthermore, the loss of the metal atoms through the devitrification and etching processes leads to the release of free halogen into the arc tube, causing arc instability and eventual arc extinction, especially in electrodeless high-intensity, metal halide discharge lamps.
Therefore, it is desirable to provide a new and improved coating for a high intensity metal halide discharge lamp that provides both chemical and thermal stability, as well as thermal compatibility with the arc tube, thereby substantially extending the useful life of the lamp.